RU2568898C1 - Method of separating polydisperse particles in micron and nanosize range and device for realisation thereof - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: separation of polydisperse particles is realised in complex plasma, in which dust component is formed from initial polydisperse powder. To create complex plasma low-temperature plasma with specified parameters is formed in discharge tube. Electric and thermal fields and particle charging are controlled by changing plasma parameters with setting parameters of selected particles. In this way, selection of particles of specified sizes and shape, their retaining in gravity field with further removal from discharge chamber are performed in complex plasma.

EFFECT: increased selectivity of separation of microparticles by sizes, as well as increased accuracy of separation with taking into account shape of microparticles.

2 cl, 7 dwg

Description

The invention relates to methods for the separation of polydisperse particles in the micron and nanoscale range and devices for their implementation used in micro- and nanotechnology, and more particularly to methods for the controlled sorting of nanopowders and composite materials.

For quantum-sized materials, including quantum dots, when materials and composites consisting of particles of the same size have unique characteristics, for example, a composition of CdSe / ZnS quantum dots with a size of 2.5 nm luminesces in the spectral range with a maximum in the region of 530 nm and quantum dots with a size of 5 nm will luminesce with a maximum in the region of 625 nm.

A known method of separating charged particles by mass [1], in which a mixture of charged particles is formed in the ionization chamber, the ionized particles are drawn by an electric field and further separation of the charged particles is carried out by a magnetic field. The separation of charged particles (ions) by mass is due to the difference in the trajectories of motion of charged particles in a magnetic field.

The disadvantage of this method is the difficulty of ionizing the source material, which requires significant energy consumption. With a single ionization, the particle will have a charge equal to the charge of the electron. With a spread in the velocities of the initial ionized particles, the trajectories of motion will differ, which will lead to the fact that particles with the same mass will not be collected at one point for separation from the mixture. The second disadvantage of this method is the large mass, as a result, the low speed of micron particles for tangible separation by a magnetic field.

A known method of separating particles by density and a device for its implementation [2], in which to separate particles by density they are placed in an electrically conductive suspension, a magnetic field is applied and orthogonal to it, which is parallel to the force of gravity. The disadvantage of this method and device is the complexity of the separation chamber. The second disadvantage is surface contamination with suspension.

A known method for separating particles of useful material and a device for its implementation [3], in which the analyzed material is subjected to irradiation with an x-ray beam with low divergence, so that the cross section of the beam is smaller than the particle size. The position-sensitive detector determines the coordinate of the particle and its separation. The disadvantages of this method are the technical difficulties of its implementation and a decrease in the size separation efficiency, since all particles having a size smaller than the transverse size of the X-ray beam will be selected.

The known method and device "Analyzers of a charged particle and methods of separation of charged particles" [4], which are closest in terms of the technical problem and the technical result achieved to the claimed method of separation of polydisperse particles in the micron and nanoscale range and the device for its implementation and adopted as prototype. The known method [4] involves the formation of a beam of charged particles flying through the analyzer, in which it is exposed to oscillating electric fields. The shapes of the fields and their intensities must be known before separation begins and depend on the type of particles. The technical result of the known method is the separation of particles by mass.

The disadvantages of the known technical solutions [4] are the high cost and complexity. In addition, the known prototype [4] requires maintaining a high vacuum and is associated with difficulty in creating a beam of charged particles.

The claimed invention is free from these disadvantages.

The technical result of the claimed invention is to increase the selectivity of separation of microparticles in size, as well as improving the accuracy of separation, taking into account the shape of the microparticles.

The specified technical result is achieved by the claimed method of obtaining the separation of polydisperse particles in the micron and nanoscale ranges, which uses a plasma-dust structure created in a plasma of a direct current glow discharge in inert gases, in which standing strata are formed by known methods. The development of complex plasma physics in the last decade provides an opportunity for its applications, but there are still no applications to the problem of powder selection [5, 6]. To separate the particles by size, it is necessary to create conditions under which particles of the same size would be grouped in one region of space for their subsequent collection.

The technical result in the claimed invention is realized by the choice of electronic temperature, the magnitude of which determines the efficiency of separation of particles. In a low-pressure glow discharge, the electron energy balance is determined by inelastic collisions with the atoms of the discharge gas. By changing the type of gas, and with it the potential of excitation and ionization, as well as the thermal conductivity of the plasma, we control the electron temperature and electric field strength in a certain range. In a dusty plasma, the electron temperature determines the charge of the dust particle.

In the used range of plasma parameters (gas pressure, discharge current, gas type), the screening length in the plasma is about 50 μm, which makes it possible to use the vacuum coupling model to calculate the charge of a dust particle.

, где l - длина, a - радиус цилиндра. В зависимости от размера и формы частиц в применяемых условиях заряд лежит в диапазоне от 500 до 50000 элементарных зарядов [5, 6].For spherical particles, q _sp = a U _fl , where a is the particle radius, U _fl is the floating potential. The charge of elongated particles q _l is estimated as cylindrical $$q_l=\frac{\mathrm{<figure-callout\; id="2"\; label="l"\; filenames="00000003.png,00000008.png"\; state="\{\{state\}\}">l</figure-callout>}}{2\ln l/a}{U}_{fl}$$

where l is the length, a is the radius of the cylinder. Depending on the size and shape of the particles under the applicable conditions, the charge lies in the range from 500 to 50,000 elementary charges [5, 6].

The essence of the claimed method for separating particles by size and shape is illustrated in FIG. 1-7.

In FIG. 1 is a schematic view of a special discharge chamber — a device for implementing the method of particle selection in a plasma. The discharge is ignited between the electrodes in the mode of existence of standing striations, the cathode is located below. Particles are introduced into the discharge from above from a container with a mesh bottom and are held in striations, which are traps of dust particles. The collecting device is brought under the vertical part of the chamber and collects plasma-collected particles on its surface. Extraction from the chamber is carried out through the lower right vacuum port.

In FIG. 2 shows the structure of the formed dust trap in a separate stratum. In the region of the head part of the stratum - at the bottom of the figure - there is an increased electron temperature and a large longitudinal electric field (index 1). The field induction vector is directed downward, then the electric force compensates for the effect of gravity on the particle.

In FIG. Figure 3 shows the dependence of the vertical forces acting on a solitary dust particle in a stratum. For a fixed particle size of mass m in a dust trap in the head part of the stratum (in the Fig. Interval 0-0.5 cm), gravity is maintained in the field. In the body of the stratum (in the Fig. Interval 0.5-1.5 cm), the electric force is not enough to hold a particle of a given mass.

In FIG. Figure 4 shows that the radial balance of forces for a particle of a certain size and shape (geometric cross section) is performed at a certain radial coordinate of r. By changing the magnitude of the thermophoresis force, one can create a balance of forces for particles of a necessary geometric shape.

In FIG. Figure 5 shows that the radial balance of forces for particles of the same specific charge, but of different shapes, is performed at different radial coordinates r. For compact (close to spherical) particles, the radial coordinate r _{1 is} less than for non-compact (elongated) r ₂ .

In FIG. Figure 6 shows the implementation of particle separation within a dust structure by form factor. At a greater radial coordinate, in the annular layer, due to the greater thermophoresis force, particles with a larger surface area are selected.

In FIG. Figure 7 shows the histograms of the statistical distributions of the initial particles - a) and extracted under the selected conditions - b). In the initial powder, the main fraction of particles has a size of the order of 1 μm; a decreasing dependence of particle sizes up to 40 μm is visible. The size of the collected powder with the selected particles varies with the original. The selected particles are in the size range from 4 to 8 microns with a maximum dispersion of 6.5 microns.

A device for implementing a method for separating polydisperse particles in the micron and nanoscale ranges comprises a housing of the discharge chamber 1, inside which a vertically oriented vacuum tube of cylindrical shape 2 with optical windows at its ends 3 and 4 and four side processes horizontally located with respect to the vacuum tube located two in the upper 5-6 and in the lower 7-8 parts of the vacuum tube, equipped with electrodes, one of which is the cathode 9 and is located in the lower process, in front of the cathode the discharge diaphragm 10 is installed, the second electrode, which is the anode 11, is located in the upper process of the vacuum tube 2, the other two processes located in the upper and lower parts of the vacuum tube 3 are equipped with ports 12-13, in the upper port 12 there is a hopper 14 with shared in shape and size by polydisperse particles, a collector 15 is located inside the lower port 13 for extracting particles separated by size and shape.

Unlike the prototype, an electrostatic trap for micron or nanoparticles is formed in the device. Particles, in contrast to the prototype, are separated statically, and not dynamically (i.e., not along trajectories during their movement). The separation of particles is carried out in the form through the use of thermophoresis.

A device for separating polydisperse particles in the micron and nanoscale ranges includes a discharge chamber, consisting of lateral processes with electrodes, feeding particles from above and collecting from below, as well as a vertical tube in which a glow discharge of inert gases is realized in a stratified mode, for stabilization and installation of standing striations narrowing aperture is used.

To implement the selection of particles of the required parameters, the electronic temperature necessary for charging the particles is set in the range of 4-10 eV and an electric field with a voltage of 0.5-20 V / cm is created to hold in the gravity field.

The claimed method was tested at St. Petersburg State University in real time.

The test results are confirmed by specific examples of implementation and are reflected in the drawings and figures in the form of corresponding histograms and dependencies.

Example 1, showing the selection of particles by size, produced under the conditions: neon gas, pressure 0.3 Torr, discharge current 1.6 mA. As can be seen from FIG. 7, in which FIG. a) presents a histogram of the distribution of the initial particles by size, in FIG. b) a histogram of the distribution of the selected particles is presented, the characteristic particle size is significantly different, in the initial distribution, the maximum falls on the size of 1 μm, while in the selected particles it is 6.5 μm.

Example 2, showing the selection of particles by size, performed under the conditions: krypton gas, pressure 0.3 Torr, discharge current 1.6 mA. In the initial distribution of particles, the maximum falls on the size of 1 μm, while in the selected particles the maximum is on the size of 4.5 μm. Compared with Example 1, conducted in neon with a larger electric field, the size of the selected particles is reduced, in accordance with numerical estimates of the levitation conditions.

Example 3 showing the selection of particles in shape. From the one shown in FIG. 6 photographs of a horizontal cross section of the dust structure trapped in a stratum shows the separation of particles having the same specific charge along the radial coordinate. The balance of radial forces for particles with a larger surface area is carried out at half the radius of the tube (annular layer), while compact particles occupy the axial region (disk-shaped layer).

During the testing of the claimed invention, it was confirmed that the spatial separation of particles in shape - sectional area - (Fig. 6); in FIG. 1 shows a general view of a particle separation apparatus.

The claimed invention is a discharge chamber with electrodes and vacuum ports, the central part of which is a vertical tube, has a diameter of 1 cm. The device itself is in a standard case, the linear size of which is 50 cm.

A device for separating particles forms dust traps, which distinguishes it from dynamic devices that separate particles along trajectories of motion.

Unlike the prototype, the claimed invention has higher efficiency due to the implementation of the separation of particles in shape.

The technical and economic efficiency of the invention consists in increasing the selectivity of particle selection due to the separation of particles by size and shape, reducing the cost of the separation unit through the use of a gas-discharge device that consumes power of the order of 1 W, which does not contain massive overall mechanical elements, which allows fine selection of particles arbitrary shape both in the micron and in the nanoscale range.

Information sources

1. RF application No. 98109222/28, "Method for the separation of charged particles by mass"; IPC B01D 59/48, Published 02.27.2000.

2. RF application No. 2007107772/03, “A method for separating particles by density and a device for its implementation”; IPC B03C 1/32, published September 10, 2008.

3. RF patent No. 2517148, "A method for separating particles of useful material and a device for its implementation"; IPC B03B 13/00, Published 05.27.2014.

4. UK patent 2,470,600 A; IPC: H01J 49/40; H01J 49/42; H01J 49/06 Published May 29, 2009 (prototype).

5. Dusty Plasmas: Physics, Chemistry, and Technological Impact in Plasma Processing / Edited by A. Bouchoule. New York: John Wiley & Sons, 1999. 408 p.

6. Vladimirov S.V., Ostrikov K., and Samarian A.A. Physics and Applications of Complex Plasmas. London: Imperial College Press, 2005.443 p.

Claims (2)

1. The method of separation of polydisperse particles in the micron and nanoscale range, which consists in placing the investigated polydisperse particles in the ionization chamber, forming charged polydisperse particles in the plasma of the ionization chamber, and then separating them in size, characterized in that before the formation of charged polydisperse particles in the plasma electronic temperature in the range of 4-10 eV and create an electric field with a strength of 0.5-20 V / cm with a longitudinal component of the electric field directed vertically down, change the electric current in the range 1-5 mA, and separated polydisperse particle form. 2. A device for separating polydisperse particles in the micron and nanoscale range, comprising a discharge chamber housing, inside which a vertically oriented vacuum tube of cylindrical shape with optical windows at its ends and four lateral processes horizontally located relative to the vacuum tube, two in the upper and in the lower parts of the vacuum tube, equipped with electrodes, one of which is the cathode and is located in the lower process, in front of the cathode there is a discharge a diaphragm, the second electrode, which is the anode, is located in the upper process of the vacuum tube, the other two processes located in the upper and lower parts of the vacuum tube are equipped with ports, in the upper port there is a hopper with polydisperse particles separated by shape and size, inside the lower port there is a collector to extract particles separated by size and shape.

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